The Organic Light Emitting Display (OLED) possesses many outstanding properties of self-illumination, low driving voltage, high luminescence efficiency, short response time, high clarity and contrast, near 180° view angle, wide range of working temperature, applicability of flexible display and large scale full color display. The OLED is considered as the most potential display device.
The OLED can be categorized into two major types, which are the passive driving and the active driving, i.e. the direct addressing and the Thin Film Transistor (TFT) matrix addressing. The active driving is also called Active Matrix (AM) type. Each light-emitting element in the AMOLED is independently controlled by TFT addressing. The pixel structure comprising the light-emitting element and the TFT addressing circuit requires the conductive line to load the direct current output voltage (OVdd) for driving.
With the progress of time and technology, the large scale, high resolution AMOLED display device has been gradually developed. Correspondingly, the large scale AMOLED display device requires panel of larger scale and pixels of more amounts. The length of the conductive line becomes longer and longer, and the electrical resistance becomes larger. Unavoidably, the power supply voltage (OVdd) will generate the IR Drop on the conductive line. The electrical resistance value of the conductive line makes that the power supply voltage obtained by each pixel circuit is different. Thus, with the same input of the data signal voltage, different pixels have different currents, brightness outputs to result in that the display brightness of the entire panel is nonuniform, and image is different, and the IR drops of the pixels are thereupon different, either.
FIG. 1 is a structural diagram of a large scale OVDD single drive AMOLED display device. The AMOLED display device is an OVDD single drive type, and comprises a display panel 1, an OVdd line 2, X direction substrate (Xboard) 3, a Chip On Film (COF) end 4. Generally, the power supply voltage in the area close to the COF end 4, i.e. the OVDD power supplying position is higher than the power supply voltage in the area away from the power supplying position. FIG. 2 is a circuit diagram of 2T1C pixel driving circuit, comprising two N-type thin film transistors T10, T20 and a capacitor C10, which is the most common 2T1C structure. The first thin film transistor T10 is a switching thin film transistor, controlled by scan signal Gate, and employed to transmit data signal Data, and the second thin film transistor T20 is a driving thin film transistor, controlled by data signal Data, and employed to drive an organic light emitting diode OLED to emit light. The capacitor C10 is a storage capacitor. The pixel driving circuit of 2T1C structure can merely function to convert the voltage into the current to drive the organic light emitting diode to emit light without any compensation function.
FIG. 3 is a brightness distribution diagram of a 55 inches AMOLED display panel. At present, the image gray scale is 255. As shown in FIG. 3, the highest brightness of the display panel is 111.6, and the lowest brightness is 88.1 In combination with FIG. 4, the highest brightness 111.6 is set to be 100% brightness, and the brightnesses of the rest positions is converted into the percentage of the highest brightness when the highest brightness is considered as the base, the lowest brightness is only 78.9%. Obviously, the brightness uniformity of the AMOLED display panel is worse. Furthermore, please refer to FIG. 5. FIG. 5 is a circuit diagram of one pixel driving circuit in the AMOLED display panel shown in FIG. 3, which comprises three N-type thin film transistors T10, T20, T30 and a capacitor C10. i.e. the 3T1C structure, wherein the first thin film transistor T10 remains to be a switching thin film transistor, and the second thin film transistor T20 remains to be a driving thin film transistor, and the additional third thin film transistor T30 receives an external signal line (monitor line), and the capacitor C10 is a storage capacitor. The pixel driving circuit of the 3T1C structure can compensate the threshold voltages of the organic light emitting diode OLED and the driving thin film transistor T20 but cannot compensate the IR Drop. Therefore, the brightness uniformity of the AMOLED display panel still remains to be worse.
In the pixel driving circuit of the 3T1C structure shown in FIG. 5, the electric compensation in the AMOLED external compensation method is utilized, which only can compensate the threshold voltages of driving the TFT and OLED but cannot compensate IR Drop; besides, the AMOLED external compensation method also comprises the optical compensation, and the optical compensation can compensate IR Drop but cannot achieve the compensation in real time. On the contrary, the AMOLED compensation method can further include the internal compensation. The internal compensation of the AMOLED is to compensate the threshold voltage (Vth) of the TFT or the channel mobility (μ) but rarely to compensate the IR drop. If the internal compensation is to compensate the IR Drop, many TFTs and capacitors have to be additionally set. The aperture ratio will be sacrificed and the necessary control signals are more.